1. Field of the Invention
The present invention relates to a power circuit for an electrical apparatus for rectifying and smoothing commercial power of AC 100 V to 240 V to obtain a direct-current voltage.
2. Description of the Related Art
Electrical appliances such as televisions, personal computers and air conditioners for use at home are designed to operate on commercial power distributed to each home.
Conventional power circuits typically used in the electrical appliances operating on commercial power include a circuit of a capacitor input type comprising a smoothing capacitor.
FIG. 1 is a circuit diagram showing an example of a conventional power circuit of such a capacitor input type.
The conventional power circuit shown in FIG. 1 comprises diode bridge 11 for performing full-wave rectification of an alternating voltage output from commercial power 10, smoothing circuit 3 for smoothing a pulsating flow voltage output from diode bridge 11, and DC-DC converter 5 for obtaining a desired voltage from a direct-current voltage output from smoothing circuit 3.
Smoothing circuit 3 comprises smoothing choke coil 20 for smoothing a current flow and smoothing capacitor 21 which is charged to smooth a voltage. One terminal of smoothing choke coil 20 is connected to an output of diode bridge 11, and smoothing capacitor 21 is connected between the other terminal thereof and a ground potential.
DC-DC converter 5 comprises switching transformer 30 for performing voltage conversion, rectifier diode 40 for rectifying output from switching transformer 30, smoothing capacitor 41 for smoothing output from switching transformer 30, error voltage detecting circuit 50 for detecting an error between an output voltage from DC-DC converter 5 and a desired voltage, error voltage transmission circuit 51 for transmitting the error in the output voltage, switching control circuit 32 for control to compensate for the error in the output voltage, and switching element 31 which is turned on/off by switching control circuit 32 for controlling a current flowing through a primary winding of switching transformer 30 to compensate for the error in the output voltage.
One terminal of the primary winding of switching transformer 30 is connected to the other terminal of smoothing choke coil 20, the other terminal of the primary winding of switching transformer 30 is connected to switching element 31, and one terminal of a secondary winding of switching transformer 30 is connected to an output through rectifier diode 40. The other terminal of the secondary winding of switching transformer 30 is grounded, and smoothing capacitor 41 is connected between the output and a ground potential.
Next, the operation of the conventional power circuit is described.
The power supply voltage of commercial power 10 is rectified in diode bridge 11, smoothed in smoothing choke coil 20 and smoothing capacitor 21, and supplied to the one terminal of the primary winding of switching transformer 30. Output from the secondary winding of switching transformer 30 is rectified in rectifier diode 40, smoothed in smoothing capacitor 41, and output as a signal with a desired voltage.
Switching control circuit 32 controls the operation of switching element 31 based on the output signal which is fed back through error voltage detecting circuit 50 and error voltage transmission circuit 51 to compensate for the error in the direct-current voltage.
Voltages of commercial power in specifications vary from country to country, and specifically, various alternating voltages ranging from AC 100 V to AC 240 V are standardized in respective countries. The standards are broadly divided into a 100 V system which employs a voltage value near 100V and a 200 V system which employs a voltage value near 200 V. It is desirable that electrical apparatuses for home use are used in common at home and overseas.
It is known that a conventional power circuit of the smoothing scheme of the capacitor input type as described above exhibits a low value in its power factor, which represents the utilization efficiency of a power source, since a current flows for a short time period near a peak voltage of the commercial power.
Thus, in recent years, since power circuits which employ the smoothing scheme of the capacitor input type are increasingly used in association with popularized electrical appliances, current of commercial power is concentrated near a peak voltage to cause voltage distortion and burning of a power distribution apparatus or the like. It is necessary to improve the power factor of the power circuits. While the use of a choke coil with a higher inductance value allows the improvement in the power factor, a more increase in the inductance value is not preferable due to problems such as increased heating and the like.
It is an object of the present invention to provide a power circuit with an improved power factor and limited heating to a relatively small amount.
To achieve the aforementioned object, the power circuit according to the present invention includes a smoothing choke coil inductance varying circuit within a smoothing circuit.
The smoothing choke coil inductance varying circuit flows a current through a secondary winding of the smoothing choke coil to provide induction based on a pulsating flow voltage obtained by rectifying an alternating voltage in a diode bridge, thereby performing such a control that an inductance value of a primary winding of the smoothing choke coil is at the maximum when the pulsating flow voltage is zero volt, is reduced with an increase in the pulsating flow voltage, reaches a predetermined value when the pulsating flow voltage is at the maximum, and is increased with a reduction in the pulsating flow voltage.
Such control of the inductance value of the smoothing choke coil enables an increase in the inductance value to enhance the improvement in the power factor when current is small and the power factor is low, and a reduction in the inductance to restrict the amount of produced heat when the current is large.
According to an embodiment of the present invention, the smoothing choke coil inductance varying circuit comprises means for determining whether the alternating voltage is of 100 V system or 200 V system based on the output voltage from the diode bridge to notify the smoothing choke coil of the determination result, and the smoothing choke coil includes means for changing its inductance value in accordance with the notification.
In this manner, since a check is made to determine whether the alternating voltage is of 100 V system or 200 V system to perform control with the inductance value of the smoothing choke coil suitable therefor, a conduction angle can be increased in a range from 100 V to 240 V to improve the power factor.